This invention relates generally to the field of inspection by a video camera in real-time of registration marks, and more particularly to a method and apparatus for identifying the correct marks among many possible marks within the field of view of an inspecting camera particularly suitable for a closed loop color to color registration system in commercial web printing apparatus.
Video inspection of registration marks is well recognized in the prior art as an important element in the implementation of automated web registration in commercial web printing environments. Of particular interest are multi-color printing machines in which successive printing operations are performed in a plurality of colors on a moving web at successive repeat lengths. In such systems it is necessary that the locations on the web at which the successive printing operations are performed have a predetermined relationship to one another. Each printing operation applies a different color in a predetermined pattern to the web superimposed to form a desired multi-color image. To obtain high quality resolution in the final product it is necessary that the patterns printed be precisely aligned. This alignment is referred to as registration
It is known in the prior art that registration of the printed pattern may be checked by printing registration or alignment marks on the printed web. This may be done, for example, by applying a mark of one color having a tolerance range and then printing a mark of another color within the tolerance range of the first mark. The operator then evaluates the registration marks and by controlling axial, circumferential, and skew adjustments controls registration. This adjustment process is very time-consuming and demanding on press operators and introduces the possibility of error as well as limiting the accuracy with which register may be controlled. Thus, automatic measurement and control of registration adjustment is highly desirable.
One approach to automatic registration control compares the location of the registration mark on the web at each printing stage and adjusts the work applying members of the printing apparatus until the locations of the marks have a predetermined relationship. Such a system employs a photoelectric sensor for detecting the position of the registration marks applied to the web.
In one prior art system utilizing a CCD video camera, a video image of the signature of the web or a portion is acquired and processed by an automatic control system to identify registration marks and their locations. Since the signature has a consistent repeat length, the approximate recurring location of the registration marks is predictable. To acquire a video image from the moving web, a high intensity, short duration illuminating source (e.g., a strobe light) may be utilized to stop the motion. However, in the prior art, the registration marks can sometimes be misidentified because more than one mark with the same detected color and correct position may be found within the field of view of the camera. This is especially a problem when special colors are used. This prevents reliable registration mark acquisition and analysis, thereby preventing reliable color to color registration.
It is accordingly an object of this invention to provide a novel method and apparatus for selecting the proper registration marks found with the field of view of the inspection camera of an automated registration system to permit reliable high speed real-time registration mark acquisition.
It is yet another object of the invention to provide a novel and reliable method and apparatus for an automated registration system in which reqistration mark scoring is utilized to increase reliability of color to color registration.
It is another object of the invention to provide a novel method and apparatus for scanning registration marks found within the field of view of the inspection camera to eliminate marks that would confuse the automated registration system.
Briefly, according to one embodiment of the invention, apparatus is provided for identifying each color component of a color-to-color registration mark comprising a pattern of a plurality of multiple element color components in a multi-color automatic registration system. For example, in the illustrated embodiment, the mark comprises a pattern of a plurality of dot pairs each having two dots with one pair associated with each color of the system. Apparatus is provided for locating each element of the registration mark, for determining position, color and size information for each element and for matching mark elements to identify each possible multiple element component based upon the position and color information. Apparatus is also provided for scoring the possible components by accumulating a score for each possible component based upon the position, color and size information and for selecting the color components to be utilized as identified color components of the registration mark based upon the scores.